1. Field of the Invention
The present invention relates to a method for producing metallic nanoparticles suitable for forming printed circuits and internal electrodes of multilayer ceramic capacitors, metallic nanoparticle produced by the method, and a dispersion containing the metallic nanoparticles.
2. Description of the Related Art
As a method for quickly forming a fine conductive pattern on a substrate, a method in which a dispersion liquid of gold or silver nanoparticles is discharged using an ink jet, dispenser, or other device to form a conductive pattern of gold or silver is known (e.g., see Japanese Patent Application Laid-Open (JP-A) No. 2002-299833). However, gold and silver are so expensive that they are not suitable for low cost production. On the other hand, copper is a low-cost material having favorable electric conductivity, and is widely used as a material for, for example, circuit members of printed wiring boards, various electrical contact members, and electrode members of capacitors. With the recent miniaturization of electronic devices, formation of multilayer circuits with a higher density, and small-size and large-capacity multilayer ceramic capacitors are required. In order to form fine conductive patterns and thin film electrodes, there is a need for a dispersion of copper nanoparticles that is easy to handle.
Examples of known methods for producing copper fine particles include a method of reducing copper oxide using a hydrazine reducing agent in an aqueous solution containing a protective colloid such as gum arabic (e.g., see Japanese Patent Application Publication (JP-B) No. 61-55562), a method of reducing copper hydroxide at a pH of greater than 12 using a reducing sugar to form cuprous oxide, and then reducing the cuprous oxide at a temperature of 50° C. or higher using a hydrazine reducing agent (e.g., see Japanese Patent No. 2638271), and a method of reacting copper oxide with a reducing agent in an aqueous solvent in the presence of a sulfur compound and a protective colloid (e.g., see JP-A No. 2004-256857).
Copper fine particles produced by these methods may be mixed with a binder such as an epoxy resin to make a paste or paint, which is printed or applied onto a substrate. The pastes or paints may be layered as necessary. The substrate having thereon a paste or paint layer is heated and burned to form an electric circuit or electrode.
However, the copper fine particles produced by the above-described method have a large average particle size of 0.05 to 1 μm, and thus are not suitable for forming a fine circuit of several tens of μm or less. In addition, the particles require a sintering process at high temperatures. Therefore, there is a need for copper fine particles having a smaller particle size.